The present invention relates to stereoscopic display apparatus in which the stereo pair images are differentiated by their state of polarization. The invention is particularly useful with respect to LCD (liquid crystal display) projectors or projection engines, and is therefore described below in apparatus using such devices.
A stereoscopic display is made up from two superimposed images, representing the left-eye and the right-eye views, respectively. In order to create a stereoscopic effect, each eye of the viewer must be exposed only to its corresponding image. There are several known methods to achieve this, one of them being known as a “passive” method. In the passive method, the two stereoscopic images are polarized in mutually orthogonal polarization states. To view the display the observer wears special binoculars with appropriate polarization filters, such that each filter transmits efficiently to each eye the light of one image and rejects the light of the other.
A block diagram of a passive stereoscopic projection system is shown in FIG. 1. The image generator provides the stereoscopic contents via two electronic signals to the two projectors. The beam of each projector is manipulated by an appropriate optical filter to ensure that the two images that are displayed by the two projectors will be polarized orthogonally to each other. The two filters can be disjoint or combined in a single unit. Another possible design is shown in FIG. 2. In this design the filters are disposed internally between the projection engine of the projector and the projection lens, and their output is combined optically. In this design there is only one projection lens. This fact facilitates the setup and the use of the projection unit.
Two types of polarization states are normally used in stereoscopic displays: linear and circular. Linear polarization is most popular because linear polarizers are most common and normally less expensive than circular polarizers. However, filtering of linearly polarized images is sensitive to the viewer's head orientation. For this reason circular polarization is favored whenever the viewing is combined with motion.
If the output light of the projector is unpolarized, it is possible to create two orthogonally polarized images simply by mounting linear polarizers on the two projectors, oriented in such a manner that the angle between their polarization axes is 90°; see for example Andrew J. Woods, “Optimal Usage of LCD Projectors for Polarised Stereoscopic Projection”, Proc. of SPIE Vol. 4297 (2001). This method is used with digital-light-processing (DLP) projectors but involves a loss of at least 50% of the light.
Many of the projectors used today are LCD (liquid crystal display) projectors. The output beam of LCD projectors is made up of three color components: red, blue and green. All three color components are linearly polarized, but the polarization direction of the red and the blue components is perpendicular to the polarization direction of the green component, as illustrated in FIG. 3. Such a beam will be referred to as “cross-polarized”.
In general, when a polarizer is disposed in the output beam of an LCD projector, it will seriously distort the colors of the image. For instance, if the polarizing axis is aligned parallel to the polarization plane of the green light, all red and blue light will be absorbed, and only the green content of the image will be transmitted. The only way to polarize linearly all three colors in the same direction while preserving their relative intensities is to align the polarizer at 45° with respect to all three colors. But this method also results in the loss of at least 50% of the light since the intensity of all three colors will be reduced, equally, by at least 50%.
One can thus use the same method used to create mutually orthogonal images with DLP projectors also with LCD projectors. The only difference is the additional requirement to orient the polarization axes of the polarizers at 45° with respect to the polarization directions of the color components. This is illustrated in FIG. 4, where the reduced lengths of the arrows of the various color components indicate the light loss incurred by the use of polarizers.
LCD microdisplays, which serve as the image-generating element in LCD projectors, need polarized light to function properly. As the light sources used for these projectors (arc lamps) are unpolarized, much of their light is wasted and the optical efficiency is reduced. This tempted practitioners in the field to invent LCD projectors that can fully utilize the unpolarized light of the source. Such projectors with transmissive and reflective LCD microdisplays were described in Atarashi et al., U.S. Pat. No. 5,172,254 and Colucci et al., U.S. Pat. No. 6,231,189, respectively. Both utilize six LCD microdisplays, two for each color. Such projectors could be used for stereoscopic display, as each can accept two full-color images. The present invention focuses on using available off-the-shelf projectors or projection engines to avoid the high cost involved in development of new designs such as those described in the patents cited above.